A small amount of jealousy is normal and, arguably, healthy in a relationship. Knowing your partner could be with someone else if they wanted to can make you appreciate them more, treat them better and not get complacent. It's a common, useful feeling, grounded in reality (a relationship is a choice: you've chosen each other).
Less useful, however, is a less-discussed (but potentially just as prevalent) phenomenon known as "retroactive jealousy": feeling curious and jealous about a partner's past relationships and sexual history, even if you know there's nothing going on. At its most innocuous, it might mean glancing at their ex's Instagram every once in a while out of boredom, or being a little too interested in what was said in their most recent "friendly catch-up". At its worst? Retroactive jealousy is obsessive, compulsive and ruinous. It means delving through the entire backlog of posts, comments and photos of them together online, constantly comparing yourself with the ex and incessantly questioning your partner about every element of their relationship.